Field of the Invention
The present invention relates to a work vehicle, and more specifically to a hybrid work vehicle and a method of controlling the same.
Background Information
Work vehicles, such as wheel loaders, are known to be equipped with power transmission devices that include a torque converter and a multi-speed transmission (termed, “torque converter transmission” below). Existing work vehicles equipped with the torque converter transmission device are provided with a clutch for switching the vehicle between forward travel and reverse travel. For instance, when a forward-reverse travel lever is placed in the forward travel position, a forward-travel clutch is connected and a reverse-travel clutch is disconnected. When the forward-reverse travel lever is placed in the reverse travel position, the reverse-travel clutch is connected and the forward-travel clutch is disconnected. Moreover, when the forward-reverse travel lever is placed in the neutral position, the forward-travel clutch and the reverse-travel clutch are both disconnected. Hereby, transmission of the drive power from the engine to the travel device is shutoff.
Whereas, in recent years, hybrid work vehicles have been proposed where the drive power from the engine and the drive power from a motor serve as the power transmission to run the vehicle instead of the torque converter transmission. For example, Japanese Unexamined Patent Application Publication No. 2006-329244 discloses a hydraulic mechanical transmission (HMT), and alternatively an electro-mechanical transmission (EMT) as the power transmission device of the hybrid work vehicles.
The HMT includes a planetary gear mechanism, and a first pump-motor and a second pump-motor connected to the rotational elements in the planetary gear mechanism. The first pump-motor and the second pump-motor function as either a hydraulic motor or a hydraulic pump depending on the travel state of the work vehicle. The HMT continuously varies the rotation speed of the output shafts to thereby change the rotation speed of the pump-motors.
The EMT uses an electric motor in place of the hydraulic motor in the HMT. That is, the EMT includes a first generator-motor and a second generator-motor. The first generator-motor and the second generator-motor function as either an electric motor or an electric generator depending on the travel state of the work vehicle. Similarly to the HMT, the EMT continuously varies the rotation speed of the output shafts to thereby change the rotation speed of the generator-motors.